Direct gravitational measurements of subatomic particles seems impossible. Has it been done/tried? The mass of the proton as I know it is always "indirectly" determined by (for example) bending a beam of charged protons in a magnetic field.
Could that be because you can gather a large number of neutrons together without them scattering in all directions before you can measure them?
Hydrogen - yes, why not? Or any other atom, for that matter.